Heretofore, metals such as stainless steel, metallic titanium and nickel/cobalt alloys and ceramic materials such as alumina and zirconia have been used as materials for living hard tissue replacements or implants such as artificial bones, dental roots, crowns and joints. For lack of assimilation to living tissue, however, they remain as useless xenobiotics in a living body after they have performed their duties. This causes an inconvenience in that such xenobiotics should be extracted and removed by surgical operations.
Calcium phosphate-based materials bearing resemblance to the composition of bones and teeth, on the other hand, assimilate to a living body with time and bond directly to the living tissue as time goes by. For that reason, no special operations are needed for the removal of them, since they cause no trouble even when remaining in the living body. Recently, ceramics based on calcium phosphate, composed primarily of apatite, tricalcium phosphate, bioglass or the like, have thus attracted attention as biologically active implant materials. However, such ceramics based on calcium phosphate has only limited use, since they are lower in mechanical strength than metals such as stainless steel and metallic titanium and ceramics based on metal oxides such as alumina and zirconia. In order to eliminate such disadvantages, various materials have been proposed, inclusive of sintered materials comprising calcium phosphate compounds containing alumina, silica and other metal oxides (Japanese Patent Publication 57(1982)-40803, crystal glass materials of high strength comprising .beta.-tricalcium phosphate, apatite and diopside crystals (Japanese Patent Application Kokai No. 61(1986)-197446), materials in which hydroxyapatite is fused onto the surface of a core material having biological compatibility and strength (Japanese Patent Application Kokai No. 63(1988)-300754) and crystallized glass materials in which reinforcing components such as magnesia and silica are incorporated into a calcium phosphate-forming component (Japanese Patent Application Kokai No. 63(1988)-303830). As a premise, however, these materials should all be basically composed of calcium phosphate compounds of the same nature as bones and teeth so as to permit them to assimilate to in-vivo living tissue. This imposes some limitation upon the range of choice of composition, and often makes it impossible to achieve sufficient improvements in physical properties. Because of its solubility in a living system, .beta.-tricalcium phosphate is likely to disintegrate before sufficient assimilation takes place. With the sintered materials in which reinforcing components such as alumina, silica and diopside are incorporated into calcium phosphate compounds, a similar problem is likely to arise, since they may be transformed into .beta.-tricalcium phosphate.
It is thus desired to eliminate the drawbacks of such conventional living hard tissue replacements and implants.